1. Field of the Invention
The present invention relates to a fiber stub in which an optical element such as an optical isolator is mounted, an optical receptacle and an optical module.
2. Description of the Related Art
An optical element such as an optical isolator for preventing reflected light from returning is generally incorporated into compact semiconductor laser modules which are mounted with high density in an optical transceiver that is utilized for optical communication. For example, Patent Document 1 and Patent Document 2 disclose a conventional fiber stub with an optical isolator, as well as an optical receptacle and an optical module which use such a fiber stub. The internal structures of these are described in reference to FIGS. 8 and 9. FIG. 8 is a longitudinal cross-sectional diagram of the main portion of a conventional fiber stub with an optical isolator. FIG. 9 is a longitudinal cross-sectional diagram of an optical module that uses the fiber stub with an optical isolator of FIG. 8.
The optical module shown in FIG. 9 is a coaxial semiconductor laser module in the case where an end portion of the optical fiber is of an optical-receptacle type. In FIG. 9, a PD (photo diode) for monitoring and lead wires for wiring the PD are omitted. This optical module is formed of an optical unit 100, which is made up of a semiconductor laser 12, a heat sink 13, a metal stem 14, a lens 15 and a lens holder 16; an optical isolator 11; and an optical receptacle 17.
In the optical unit 100, the semiconductor laser 12 is mounted on the heat sink 13 by means of soldering. The heat sink 13 is also secured onto the metal stem 14 by means of soldering. The lens holder 16 made of metal is resistance welded onto the metal stem 14. The lens 15 is secured to the inside of the lens holder 16 by means of low melting point glass or the like.
An optical fiber 1 is secured, by means of an adhesive, in a ferrule 2 made of ceramic or glass material which has a through hole 27 in the center. A fiber stub 3 is composed of the optical fiber 1 and the ferrule 2. An end of the fiber stub 3 is inserted into and secured to a through hole 28 of a metal holder 5. An end surface 4 of the fiber stub 3 is polished to incline by 8° in order to suppress light return to an optical element 12, such as a semiconductor laser, due to a near end reflection.
In order to prevent the light return to the optical element 12 more efficiently, an optical isolator element 9 is secured to the end surface of the fiber stub 3. The optical isolator element 9 is composed of a polarizer 6, a Faraday rotator 7 and an analyzer 8, which are attached to each other by means of an adhesive so that the transmitting polarization plane of the polarizer 6 and that of the analyzer 8 cross each other by an angle of 45°. In addition, the optical isolator element 9 has been cut out so as to be placed inside of the outer diameter of the end surface 4 of the fiber stub 3, and is secured to the end surface 4 of the fiber stub 3 by means of adhesion. The optical isolator element 9 is mounted and secured so that the transmitting polarization plane of the polarizer 6 becomes perpendicular or lateral in relative to the direction of the maximum inclination of the end surface 4 (the surface polished to incline by 8°) of the fiber stub 3.
A cylindrical magnet 10 applies a magnetic field to the Faraday rotator 7 in the optical isolator element 9. The magnet 10 is secured to metal holder 5 by means of adhesion so that the end of fiber stub 3 can be inserted into the inside thereof. The optical isolator 11 is composed of the optical isolator element 9 and the magnet 10.
Meanwhile, a rear end surface 24 of the fiber stub 3 is polished to have a curved surface so as to be fitted with the end of the plug ferrule of an optical connector, which is not shown in the drawing. The rear end side of the fiber stub 3 is covered with a sleeve 18 that is in hollow cylindrical form and made of ceramic or metal. In addition, the sleeve 18 is surrounded by a shell 19. The shell 19 is made of metal, plastic or the like, and is inserted and secured to the metal holder 5. The fiber stub 3, the metal holder 5, the sleeve 18 and the shell 19 compose the optical receptacle 17.
Schematically, optical modules are assembled as follows: First, the optical unit 100 and the optical receptacle 17 to which the optical isolator 11 has been attached are prepared. Next, the position of the optical receptacle 17 relative to the optical unit 100 is adjusted so that light emitted from the semiconductor laser 12 is focused on the optical fiber 1 via the optical isolator 11 by means of the lens 15. After that, the metal holder 5 of the optical receptacle 17 is secured to the lens holder 16 by means of laser welding or the like. Here, a counterbore portion 21 is created inside the lens holder 16 within the optical unit 100 so as to provide a space which is large enough for the optical isolator 11 not to make contact with the inner wall of the lens holder 16 at the time when the position of the optical receptacle 17 is adjusted.    [Patent Document 1] Japanese Unexamined Patent Publication No. 2000-162475    [Patent Document 2] Japanese Unexamined Patent Publication No. 2002-158389